1. Field of the Invention
The present invention relates to a method for automatic identification of objects or materials, for example plastic materials. By identification, is meant the extraction of information relating to the material or to the object.
This method is notably applicable to the sorting and recycling of materials originating from used objects.
2. Description of the Related Art
Methods for automatic identification of objects or materials are known, consisting of including in these objects or materials small concentrations of substances having specific luminescent properties, of irradiating them with a light beam with a wide frequency spectrum, of carrying out a spectrophotometric analysis of the response of the substances included in the material and of identifying them depending on these responses.
In their application No. 06 04578, the Applicants have for example proposed a method in which the spectrophotometric analysis notably includes the following steps, after irradiation of the object or of the marked material:                sending the waves transmitted or reflected by the object or the material onto a dispersing element which deflects them so as to obtain a light spectrum of the light intensity at different zones of the spectrum corresponding to different wavelength ranges,        detecting the light intensity in each of said zones,        comparing this intensity with one or more threshold values specifically allocated to this zone and which have been stored in memory beforehand,        the result of this comparison contributing towards determining the identity code of the material.        
However, this method is intrinsically limited in the capacity of coding information relating to the material because of the unicity of this type of excitation. Now, it may be useful to code several types of information relating to a material, for example its composition, its recycling route, its manufacturer.
It is also limited in the case of strongly colored or black materials which are relatively frequent. Coloration is due to the presence within the material of colored pigments, notably carbon black, in variable proportions. Carbon black is used as a protective agent against radiations, mainly UV radiation, in outdoor applications or as a stabilizing and strengthening agent. Its action mainly consists of absorbing the radiations received by the material which may cause degradations of the polymeric chains. However, it also has the property of absorbing radiations which may be notably emitted in the visible spectrum by the material making up the object and/or the included markers, which explains its dark or black color. The result of this is that excitation by a light source does not cause any spectral emission allowing easy extraction of information relating to the material with very small concentrations of markers if the latter is strongly colored or black.